Knee OA is a painful disabling condition that has major societal impact yet has no disease-modifying therapies. It is, therefore, salient that a rationale exists for OA-modifying effects from intra-articular corticosteroids (IACS). IACS are in clinical use for sporadic knee OA pain flares, but the notion that they might influence OA progression is new, and derives from the recent observation of a high prevalence of synovitis in OA knees. OA synovitis is more focal and less intense than that of systemic polyarthritis, but the profile of cytokine expression is similar and its presence predicts cartilage damage. Corticosteroids reduce progression of joint damage in rheumatoid arthritis, so IACS should also reduce cartilage damage in OA that is mediated by synovitis. Moreover, IACS have protective effects in chemically-induced, mechanically-induced and inflammatory animal models of OA. Our hypothesis is that IACS mitigate inflammatory damage to cartilage and peri-articular bone, and that this benefit outweighs any anti-anabolic effects on these structures. The recent development of quantitative technologies that directly image joint cartilage (MRI), measure subchondral bone properties (DXA and MRI), and evaluate synovitis in a clinical setting (ultrasonography), now allows us to test for the first time the effects of IACS on the structural progression of OA. The overarching goal of this proposed research is to test the potential for disease modification of synovitic knee OA by IACS through a clinical trial directly measuring its effects on cartilage and subchondral bone. We will enroll 140 individuals with knee OA and synovitis (confirmed by ultrasonography), into an adaptive 2-year randomized double-blind clinical trial of intra-articular triamcinolone 40 mg vs. placebo, every 3 months, administered with ultrasound guidance. We will measure changes in cartilage volume, peri-articular bone using MRI and DXA and trabecular morphometry. We will evaluate changes in symptoms, functional status and healthcare utilization using validated questionnaires and objective physical function tests. We will use mixed effects regression models for longitudinal repeated measures to test for a difference in the trajectory of cartilage volume over time between the treatment groups. We will conduct an adaptive interim analysis after the first half of participants has completed the trial. This will allow the trial to be stopped early for either success or futility, or otherwise continue.